One task that arises in the use of cardiac rhythm management devices, including pacemakers, resides in the determination of lead impedance, that is, the effective resistance of the lead wire coupling the cardiac rhythm management device to the heart via electrodes within the heart. The measured value of lead impedance can provide useful information, such as enabling the estimation of remaining battery life, and data concerning the condition of lead wires
For example, a relatively low lead impedance value may indicate a short circuit between the pacing electrodes. A relatively high lead impedance value may indicate an open circuit such as, for example, what might result from a lead wire that has become disconnected from the cardiac rhythm management device. To ensure effective pacing therapy is continuously available, each of these defective lead wire conditions must be detected and remedied as quickly as possible.
It is possible to calculate lead impedance based on a measurement of the voltage droop from a capacitively coupled pacing pulse delivered to the heart. Thus, traditionally, lead impedance measurements have been obtained by using a droop amplifier to measure the droop voltage, or a portion of the droop voltage, which appears across the pacing supply capacitor during delivery of a pacing pulse. In currently available products, the droop amplifier output is digitized by an analog-to-digital (A/D) converter, and the resulting value is transformed into an impedance value using a logarithmic function or a look-up table.
Different conditions surrounding the cardiac rhythm management device may require the use of different initial pacing voltage amplitudes. However, to accurately measure the lead impedance and compensate for changes in amplitude, either the gain of the droop amplifier must be adjusted, or the function/look-up table entries must be redefined. The first option is often chosen, since it may also be desirable to adjust the gain of the droop amplifier to allow full utilization of the AID converter's dynamic range. However, choosing to adjust the droop amplifier gain often means that a different gain value will be needed for measuring the lead impedance at each different pacing voltage amplitude. Additional software, and/or hardware circuitry may be required to provide the necessary gain adjustment. Further, adjusting the gain may introduce inconsistency into the impedance measurement as the pacing voltage amplitude varies. Thus, there is a need in the art to measure lead impedance in a manner which is substantially independent of the initial pacing voltage amplitude.